form under pressure of sediment layers and metamorphic
pressure and/or due to changing temperatures, none of them actually
melts and turns into hot, liquid magma. Igneous rocks do.
By Greg Bishop via Flickr.com
As part of the rock cycle, rocks are built, eroded and some end up
being pushed deep down
interior of the Earth. The deeper down
they go, the hotter are temperatures - the core of the Earth is very
As the conditions (pressure and temperature) change, chemical reactions
start happening, because rocks, as other things, consist of chemicals.
All chemicals strive to reach stability so chaniging conditions make
them to react to become more stable in the new conditions.
chemical reactions happen, the mineralogy of the rock changes. If that
happens in the solid state - metamorphic rock is formed.
Igneous rock, on the other hand, is formed after the previous rock has
undergone such extreme temperatures and pressures, that it has melted
and turned into hot liquid. When that liquid is under the
surface, it is called magma. When it sprinkles out of a volcano, it is
Igneous rock is formed
magma, or lava, freezes
and turns into a rock. That happens when the rock is
the Earth's surface again, and the temperatures and pressures decrease;
or, when lava sprinkles out of a volcano and turns from a liquid to a
That is also when the minerals in the rock are formed. Different
minerals form at different pressures and temperatures. That's why
different rocks have different mineralogical composition.